Neurophysics: a glimpse at the future
Physics is one of the oldest academic disciplines and has historically been interwoven with biology, chemistry, philosophy and mathematics. Although less obvious at first glance, physics has also completely revolutionised the field of neuropsychology. Physics is the study of matter, energy and force through time and space. It has shed light on invisible forms of energy like magnetism and radiation that can penetrate matter and reveal new aspects previously unobserved. In the context of neuropsychology these principles have transformed the way in which we explore the human brain.
The physicist Wilhelm Röntgen was credited with developing the use of electromagnetic radiation (such as x-rays) - he won the first Nobel Prize for Physics in 1901 for this contribution. Computed tomography (CT) scans have been developed from the basic x-ray to provide information on brain structure, especially for indicating the presence of blood or tumours. During a CT scan a series of x-rays are taken from different angles and 'matter' in the brain blocks the electromagnetic radiation to provide a gross representation of structure.
Magnetic resonance imaging (MRI) produces images with much higher spatial resolution. MRI uses the principle of physics to manipulate electromagnetic waves and detect induced changes in cortical activity. For example, during MRI rotating electromagnetic fields cause hydrogen atoms, contained within water in the body, to align in a certain way and then relax (the basis of the MRI image). Similarly, functional MRI (fMRI) can help neuropsychologists understand which brain regions are involved in a given process (for example memory) by measuring changes in cerebral blood flow during task performance. Increased blood flow in a given brain region indicates higher involvement in the task.
Whereas fMRI has a time resolution of tens of seconds an electroencephalogram (EEG) has a time resolution of tens of milliseconds. Given that it can take less than 150 milliseconds for electrical activity to travel across a neuron this means that EEG is a useful neuropsychological measure. During EEG electrodes are placed at different scalp locations to record electrical responses generated by groups of neurons. For instance, an EEG can be conducted while a person views images that depict different emotional situations. Increased activity at an electrode may signal enhanced activity in the underlying brain region. The role of physics in EEG is the detection of time-locked changes in cortical activity. Although fMRI and EEG can produce 'snapshots' of brain activity during task performance this does not necessarily mean that this brain region is directly and solely responsible for certain types of cognitive processing.
To this end, transcranial magnetic stimulation (TMS) uses the principles of physics to stimulate small regions of the brain with a mild current running across a magnetic conductor, temporarily 'knocking out' brain activity in a given region. If task performance is impaired during TMS this strongly implies that the region stimulated is directly involved with the task at hand. Although there are questions about the accuracy of TMS (with regards to the precise region(s) being stimulated) and the long-term effects of this technique, it is important to acknowledge that TMS is an important transition from simply describing the brain to changing how it responds.
So what more can we expect as physics continues to become further entwined with neuropsychology? One of the most recent applications of physics has been the introduction of 'optogenetics', a technique that uses lasers (developed by physicists) to 'probe' neurons in the brain at very high speeds. Some scientists believe that techniques like optogenetics and TMS could, in theory, one day help alter unresponsive brain regions in patients who suffer from disorders like Parkinson's disease, which causes impairment to motor skills and speech production.
Despite the fusion of these disciplines and the advances that have been made there are many outstanding questions. For example, in the brains of people who suffer from Alzheimer's disease, a neurological disorder that causes a loss of long-term memory and an inability to form new memories, scientists often find neuronal 'tangles' - but there are outstanding questions about the degree to which these neuronal tangles play a primary or more peripheral role in Alzheimer’s disease. Imagine a future, then, where physics and neuropsychology were completely intertwined, with physicists and neuropsychologists working alongside one another on a daily basis in the laboratory. What would that future hold? A continuing collaboration between neuropsychology and physics is likely to help answer such outstanding questions about the human brain and make today's impossibilities tomorrow’s practical reality.